Currently QML object constructors only fake inheritance by calling the constructor of the superclass. By using prototypes inheritance should be implemented the real javascript way. Moreover it would bring huge performance improvements, by creating the properties in the prototype.
This project is unfortunately no longer maintained. If a new maintainer wants to step up and take care, the project is archived here: https://invent.kde.org/unmaintained/qmlweb You can just clone it in your private namespace on invent.kde.org and if you have started to work on it and fixed/implemented something get it reviewed and the project unarchived. Sorry for the inconveniences.